1. Field of the Invention
The present invention relates to multiple data rate baseband receivers which are used to detect the presence of, and to recover bipolar or alternate mark inversion (AMI) data and timing information from a transmission medium such as a transmission cable. The cable may be of short or long length and the data may be transmitted at a plurality of data rates.
2. Description of the Prior Art
In digital data telephone transmission systems there are a plurality of different data rates commonly used for data transmission. For example, in the dataphone digital system of ATT there are four different data rates used, i.e., 2400 baud, 4800 baud, 9600 baud and 56,000 baud. Alternate mark inversion (AMI) data is transmitted over existing voice grade telephone wires between a telephone central office and the subscribers. In systems of the prior art, separate receivers or at least separate circuitry are typically required for each data rate transmitted, particularly with respect to equalization.
Baseband receivers require some form of equalization to compensate for distortion in the transmitted signal by the transmission cable. Clock recovery circuitry is also required to recover timing information from incoming data. Additionally, no signal detection is used to detect the absence of an incoming data signal.
The equalization required to recover the transmitted data varies with cable length, type and environmental surroundings. To eliminate an infinite number of equalizer designs and/or adjustments, the equalization is typically adjusted partially or completely automatically by receiver control circuits based on incoming signal amplitude and knowledge of the transmission cable characteristics, data rate and the transmitted pulse amplitude.
Timing information is typically recovered in the prior art by a tank circuit which is excited by transistions in the incoming data or by a phase locked loop circuit which will lock with the proper phase relationship to the incoming data.
No signal detection is typically accomplished in the prior art by monitoring the automatic equalizer control circuitry and detecting an attempt to recover a signal which is smaller than that which is possible based on knowledge of transmitted pulse amplitude and maximum cable attenuation limits. An alternate method of no signal detection is to detect that the tank circuit used for clock recovery is not ringing which indicates that insufficient energy at the data rate is coming into the receiver and therefore no data signal is present.
These prior art techniques for data recovery are based on a given data rate. The equalizers are designed for data signals over a narrow frequency range and will not properly compensate for significantly differing data rates. The clock recovery techniques are also suited only for a specific data rate due to the operation of the tank circuits or phase locked loop filters and dividers.